New equalizers have recently been developed to specialize in music audio signals. Such new technique is called as a musical instrument equalizer which is capable of manipulating the volume and replacing the timbres of individual musical instrument parts. While equalizers installed inmost of audio players change musical sounds by manipulating the frequency range, musical instrument equalizers change musical sounds by manipulating the individual musical instrument parts. Such musical instrument equalizers are expected to expand the scope of music appreciation. The music instrument equalizer of Yoshii et al. called Drumix, as shown in non-patent document 1, successfully manipulates the volume and changes the timbres of percussive instruments such as snare and bass drums. The music instrument equalizer of Itoyama et al., as shown in non-patent document 2, is capable of manipulating the volumes of all musical instrument parts including percussive instruments. Unlike Yoshii's Drumix, however, Itoyama's equalizer does not manipulate the timbres of musical instrument parts. An invention based on non-patent document 2 has been included in PCT/JP2008/57310 as identified WO2008/133097 (patent document 1).